A feedback amplifier is a simple design approach for broadband gain stages where noise figure and power efficiency are not a primary driver. Four variations of a simple one stage feedback amplifier were designed using a 0.13 um GaAs Pseudomorphic High Electron Mobility Transistor (PHEMT) process from TriQuint Semiconductor. The design and fabrication of these circuits was performed during the Fall 2013 Johns Hopkins University Monolithic Microwave Integrated Circuit (MMIC) Design Course, taught by the author. In these very compact amplifier designs, an external bias was required for the drain supply. A modification to the feedback designs to include a broadband DC supply using a second PHEMT as an active load is also presented, both simulations and layouts.

Resistive Feedback Broadband Amplifier

One way to achieve broadband gain with an inverting transistor, such as a GaAs MESFET or PHEMT, is to use resistive feedback to achieve octaves or even a decade of bandwidth. Figure 1 shows the simplest schematic of the feedback amplifier with the two key components that can be tuned for the desired return loss, stability, and gain characteristics; the value of the feedback resistor, and size of the transistor. In this case, a MMIC PHEMT can be varied in total periphery (size = width of gate fingers * number of gate fingers). This simple arrangement ignores the DC bias, but it is a good starting point to simulate the small signal performance without worrying about bias yet.

Figure 1 • Simplest Schematic of the Feedback Amplifier.

If you only have a non-linear model, then it is a good time to add a large capacitor to DC block the drain bias from the gate bias in the feedback path. A 4 pF capacitor was added to the resistive feedback path for these designs. Two basic feedback amplifier variations were created, one used a standard 6x50um PHEMT, and the other a smaller 4x38um PHEMT. For this process, a large shunt resistor on the gate will provide part of a broadband bias with VGS=0V, while the drain bias was expected to be an external bias tee for these compact layouts; about the size of a PHEMT layout for probe testing (see Figure 2).

Creating symmetry in microwave circuit layout is typically desirable. A new design was laid out with the RC feedback path split into two dual parallel paths which forced a longer connection to ground. This additional source inductance was reduced by using two parallel substrate vias (Figure 3). So while the single RC feedback versus the symmetric dual parallel RC feedback design is not quite an “apples to apples” comparison, they both resulted in broadband stable amplifier designs.

Both the symmetric and single RC feedback amplifiers resulted in nearly identical performance. Figure 4 shows the measured s-parameters and noise figure (green) of the 6x50um PHEMT based amplifiers, resulting in a gain of 16 dB at 1 GHz that gradually drops to 8 dB at 15 GHz. The noise figure was about 1.5 dB from 1 to 6 GHz, gradually rising to 2 dB at 15 GHz. Another pair of the single and symmetric RC feedback amplifier designs was created using a PHEMT of half the size, e.g. 4x38um, which consumes half the DC power of the 6x50um design(s). Performance was similar, though the noise figure was slightly higher and the gain slope falloff is more gradual as shown in Figure 5.

For the 4x38um amplifier, the gain was measured as 14 dB at 1 GHz gradually dropping to 9.5 dB at 15 GHz. The noise figure was about 1.7 dB from 1 to 6 GHz, gradually rising to 2.1 dB at 15 GHz. These measured results agreed well with the original Microwave Office (MWO) analytical simulations, as well as EM simulations using Axiem, Momentum, and Sonnet. Figures 6 and 7 compare the gain (magenta) and noise figure (blue) measurements (solid) versus simulations (dotted) for the 4x38um and 6x50um feedback amplifiers. The “break” in the noise figure measurement at 6 GHz is due to using two different instruments for the noise figure measurements, one up to 6 GHz, and the other starting at 6 GHz.

Biasing of the original broadband feedback amplifiers assumed that the drain DC bias was provided through an external bias tee. The gate bias for these devices was already broadband, supplied by a large shunt resistor (2K) to ground, since these PHEMTs perform well with VGS=0V. So how do you add a broadband biasing circuit? One solution that keeps the layout compact and provides a bias that is tolerant to variations in processing, is to use a second PHEMT as an active load to bias the amplifier. The drain voltage can be supplied and split across two equal sized PHEMTs.

Figure 8 shows a simple schematic of the feedback circuit which now has an active load, and two additional capacitors. At the input pad for the drain voltage (Vdd), a shunt cap (4 pf) to ground isolates the RF match from the external DC connection. A second capacitor (4 pf) is used to DC block the drain voltage of the amplifier from the RF output. The size of these capacitors is a tradeoff of size versus the low frequency rolloff of the gain. Also, the active load could be changed to be smaller or larger than the PHEMT used for the amplification. A smaller active load reduces the current consumption and lowers the noise figure, but makes an unequal split of the Vdd supply voltage, reducing the voltage swing or output power of the amplifier. Conversely, a larger active load increases the current consumption but increases the proportion of Vdd split between the two PHEMTs, thus improving efficiency but with an increase of noise figure. Simulations were performed using an active load of 60% and 150% of the nominal 4x38um or 6x50um size used in the feedback amplifier with little change in the small signal performance, but with a small effect on noise figure.

For example, active loads of 6x30um, 6x50um, and 6x75um, with the 6x50um feedback amplifier resulted in a DC bias ranging from 2.1V at 20 mA, to 3V at 27mA, to 3.7V at 33mA with a 6V Vdd supply. The gain did not change much, but the noise figure at 3 GHz simulated over a range of 1.8 dB, 2.0 dB, and 2.3 dB over these same active loads as the drain current increased. Note that the active load provides a small compact broadband DC bias, but does increase the noise figure from the 1.4 dB in the original designs that required an external drain DC bias.

The use of an active load to bias the broadband feedback amplifier results in a very compact layout, though there is some increase in noise figure, and a rolloff of low frequency gain below 1 GHz, plus a slight drop in gain across the whole band. Figure 9 shows the simulated s-parameters of the broadband DC supplied feedback amplifier (solid) versus the original design (dotted) using an external bias tee. Figure 10 shows the compact layout with the addition of a Vdd pad (nominally 6V), and the same ground-signal-ground (GSG) probe test RF input and output. Another advantage of the active load is that using another PHEMT for the DC bias makes the amplifier design robust to process variation.

Feedback amplifiers can provide very broadband gain with moderate noise figure and efficiency, in a simple compact layout. One simple approach to supplying DC bias over a broadband is to use a PHEMT active load. This approach can also be very small but will have some negative impact on the noise figure and efficiency. For any application requiring a simple broadband gain block where noise figure and efficiency are not primary drivers, feedback is a good approach for the design tradeoff of bandwidth, gain, return loss, stability, noise figure, and efficiency.

Acknowledgements

I would like to acknowledge the support of TriQuint Semiconductor for fabricating designs for JHU students since 1989. Software support for these JHU designs is provided by Applied Wave Research/National Instruments (AWR/NI), Keysight Technologies, and Sonnet Software.

About the Author

John E. Penn received a B.E.E. from the Georgia Institute of Technology in 1980, an M.S. (EE) from Johns Hopkins University (JHU) in 1982, and a second M.S. (CS) from JHU in 1988. Since 1989, he has been a part-time professor at Johns Hopkins University where he teaches RF & Microwaves I & II, MMIC Design, and RFIC Design. Email: profpenn@gmail.com.

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